This invention relates to an improved method and apparatus for lining reservoirs, waste landfills, hazardous waste disposal sites, outdoor fluid containment areas and other similar applications. Specifically, the present invention relates to a moisture impervious thermoplastic sheet or liner particularly suitable for environmental pollution control as a water barrier for the building of waste landfills, ponds, reservoirs, lagoons and as a soil sealant for hazardous or nuclear waste and capable of resisting expansion, contraction, wrinkling, bridging, deformation, and stress cracking due to thermal changes in the plastic sheet upon exposure to direct sunlight or incident radiation. This invention also has the stated advantage of reducing desiccation of the subgrade clay layer below the liner caused by heating of the liner, thereby increasing liner longevity.
Darkly pigmented plastic or thermoplastic sheets or films have been utilized to seal the soil at landfills or hazardous waste sites by laying the plastic sheeting along the ground with the seams overlapped and welded or cemented by known methods. Various bentonite-containing sheet barriers also exist for use as liners for extremely large areas such as ponds, lagoons and hazardous waste sites.
Landfills, lagoons or other waste ponds are typically constructed by excavating land to create a reservoir area. If desired, berms can then be built around the perimeter of the reservoir area to extend the walls of the reservoir above ground level. Quite often, the landfill, lagoon or waste site is next lined with a layer of clay to serve as a barrier. For example, environmental regulations typically require a subgrade clay layer of uniform thickness (e.g., five feet) and uniform water content. As a final step, a thermoplastic liner is installed by, for example, placing dark-pigmented high density polyethylene ("HDPE") or dark-pigmented medium density polyethylene ("MDPE") plastic sheets over the entire surface of the reservoir soil and berm area in an overlapped abutting fashion, and then welding or cementing the sheets together to create a water impermeable liner. An exemplary method of welding plastic materials, such as plastic shetting, is et of forth in U.S. Pat. No. Re. 32,103 assigned to Gundle Lining Systems Inc. The liner can then be covered, if desired, with a protective layer of earth to provide protection of the liner from puncturing and to help keep the liner in place.
The waste, liquid, sludge material or the like is then placed on top of the plastic sheeting which is lining the landfill, reservoir, lagoon or pond. The landfill, reservoir, lagoon or pond is subject to water or fluid level changes, thereby leaving the dark-pigmented plastic liner vulnerable to direct exposure to sunligh, particularly along the sidewalls of the landfill or berm areas. Exposure to the sunlight causes the dark-pigmented plastic liner to heat up and winkle, buckle, bridge, or deform due to the thermal characteristics of the plastic. Such wrinkling or buckling causes stress to be placed on the plastic sheets and the seams between each plastic sheet, thereby damaging or potentially damaging the seal created by the liner. In addition, the higher temperatures caused by black absorbance of infrared radiation accelerates stress cracking. Thus, thermal stress cracking of the plastic liner is an existing problem in the hazardous waste liner industry. The applicants have determined that stress cracking is greatly accelerated as the temperature of the liner increases (i.e., for approximately every 11.degree. F. increase in temperature, the stress crack growth rate doubles).
The polyethylene lines are darkly-pigmented because they contain carbon black of sufficiently high concentration (i.e. approximately 2-3%) to help prevent damage to the plastic due to incident untraviolet radiation from the sun. However, the dark pigmentation absorbs heat thereby causing thermal damage to the plastic. Stress cracking is initiated and stress cracks are propagated in a HDPE layer under the stress of heat and tension.
Additionally, the heat generated from exposing the dark-pigmented plastic to incident radiation causes the underlying clay layer to dehydrate and crack, thereby causing damage to the functional and structural seal integrity of the lagoon, reservoir or pond. Desiccation of clay subgrades is a current problem in landfill or waste site construction. This subgrade desiccation dehydrates the subgrade clay layer near the top surface of the containment area berm. The soil moisture evaporates under a temperature gradient into air pockets existing the underside of the plastic liner and the clay. The soil moisture then condenses on the underside of the plastic liner and drains down-gradient on the berm slope toward the bottom of the berm. The subgrade clay layer no longer maintains uniform moisture content and will continue to develope cracks and fissures which destroy the desired impermeability of the clay layer. Thus, the plastic liner must be removed so that the clay layer can be restored to the required uniform moisture content. Such restoration efforts are costly and time consuming.
Therefore, particularly where hazardous wastes are involved, the appearance of any abnormalities in the plastic liner or clay layer of the waste pond will cause concern that the waste containment area is not functioning properly and will likely result in substantial time and money investments to alleviate such abnormalities.
Heretofore, the plastic sheets used for lining waste sites, ponds or lagoons were constructed out of a dark-pigmented plastic material, such as HDPE. Such plastic sheeting is available in varying thickness, width, and length. For example, Gundle Lining Systems, Inc., Houston, Tex., provides 250 foot long rolls of 100 mil thick, 22.5 feed width, dark-pigmented high density polyethylene plastic sheeting under the label "Gundline HD" for use in such applications.
Additionally, the exposed surface of the plastic sheeting can also be roughened, for example, by creating turbulence in the outer layers of the sheet during manufacture, leaving asperities in the sheet surface making it rough and therefore skid-resistant for those walking on the surface.
The dark pigmentation of the HDPE plastic liners posses desirable characteristics, such as, a carbon black content and distribution which is useful in blocking ultraviolet initiated oxidative degradation. For example, carbon black concentration inhibits the formation of free radicals and carbonyl groups which couild otherwise form in polyethylene upon exposure to ultraviolet light, thereby preventing the free radicals and carbonyl groups from catalyzing the chain reaction breaking of the polyethylene bonds. However, the dark pigmentation of this HDPE plastic sheeting acts like a solar collector, absorbing sunlight or incident radiation thereby building up heat and causing damage to the plastic sheeting. While exposed plastic liners can be protected from sunlight by covering the liner with dirt, this procedure may not be desirable or practical, and may limit the ability to visually inspect the exposed portions of the liner for any tears or other undesirable perforations or leaks. Sections of lined landfills are typically left exposed until the section is ready for waste disposal which may not be for several years. Even if a layer of dirt is placed on the exposed plastic liner, this dirt may become dislodge and washed away over time.
HDPE liners are highly desirable to the waste containment industry primarily because they possess desirable chemical resistance and strength. However, the use of HDPE reduces the liner's flexibility and elasticity thereby reducing the liner's ability to absorb stress and resist cracking. More flexible liners such as very low density polyethylene ("VLDPE") are therefore attractive in certain applications, but if used alone, would compromise the chemical resistance and strength of the liner.
HDPE, MDPE and VLDPE are available in pellet form from a variety of manufacturers. It should be noted that there is not presently an American Standard Testing Material (ASTM) standard for VLDPE. ASTM standard D 1248-84 provides that polethylene is identified primarily on the basis of density and flow rate. This ASTM standard also notes the present categorization of polyethylene density (as calculated by ASTM testing protocols): Type I--"Low Density" (0.910 to 0.925 gram/cm.sup.3); Type II--"Medium Density" (0.926 to 0.940 gram/cm.sup.3); Type III--"High Density" (0.941 to 0.959 gram/cm.sup.3); and Type IV--"High Density" (0.960 and higher). It should be further noted that in the plastics industry, a polyethylene may actually be called "High Density" without actually fitting the ASTM standard. As used herein, the term HDPE is intended to include polyethylene having a preferred density range of between 0.93 and 0.96 gram/cm.sup.3 (as calculated by ASTM procedures) though higher density values may be utilized. As used herein, the term VLDPE is intended to include polyethylene having a preferred density range of between 0.89 and 0.92 gram/cm.sup.3 (as calculated by ASTM testing procedures) though lower values may be utilized. As used herein, the term MDPE refers to polyethylene having a density falling between HDPE and VLDPE densities.
Additionally, it is difficult to visually detect any tears, rips, cracks, perforations, punctures or other potential leak forming channels in the surface or seams of dark-pigmented plastic sheeting. Furthermore, the dark-pigmented plastic sheeting does not accept paint, thereby preventing or inhibiting the use of a light-reflective paint on the exterior of the plastic sheeting to prevent the heat buildup when the sheeting is exposed to sunlight.
In accordance with the present invention, a method and apparatus are provided which will allow formation of an improved fluid impenetrable flexible plastic sheet or geo-membrane which will sufficiently reflect sunlight or incident radiation in order to resist thermal expansion, contraction, and cracking when exposed to direct sunlight, will arrest crack propagation, will inhibit subgrade desiccation, and will be suitable for use over large outdoor fluid containment areas by covering the soil or other areas with the improved plastic sheet.